The invention relates to a multi-wavelength laser, and more particularly, to a modulatable external cavity fiber laser providing stable multi-wavelength laser emission with a predetermined wavelength spacing for wavelength division multiplexing applications.
Optical networks are becoming increasingly complex and use multiple wavelengths transmitted simultaneously over the same fiber. This transmission method is referred to as dense wavelength division multiplexing (DWDM), with the number of active channels continuing to increase. The international telecommunications union (ITU) standards body has proposed a channel allocation grid with 100 GHz channel spacing (xcx9c0.81 nm at a 1550 nm wavelength) on even 100 GHz intervals, counting nominally in both directions from a center frequency of 193.1 THz. Newer systems are being designed to reduce the channel spacing to 50 GHz or less. In addition, the total wavelength range over which these devices are designed to operate is increasing. Cost control is very important for system deployment. It would be very desirable to have one laser source module serve as the source for all the channels in the optical system. In addition, it is desirable to have the module be both scalable and upgradeable in a granular manner. Scaleable means that the system may be upgraded from small channel counts to larger channel counts. Granularity implies that the upgrades can be implemented one channel, or a few channels, at a time.
Optical communication systems are required to have a service life in excess of twenty years. Lasers should be easily reconfigurable and/or replaceable if one of the laser channels malfunctions. In addition, as channel count increases from the present channel count of less than 40 to greater than 100, it would be more cost-effective not to stock 100 different spares, but preferably at most a few xe2x80x9cgenericxe2x80x9d lasers that can be used for several wavelengths.
With a channel spacing of 25 GHz or less, wavelength stability of laser channel sources must be better than a few percent of the channel spacing. The component and system cost of new generations of networks that require greater stability and closer channel spacing should also not increase. Furthermore, crosstalk between channels should be less than 30 dB of the peak channel power which is typically in the range of 1-10 mW.
It would be advantageous to have a single laser module serve as the source for all channels in an optical system. This is a difficult requirement because of mode pulling and homogenous broadening of lasing transitions in gain media. Multi-mode lasing has been demonstrated in erbium-doped fiber ring lasers using an intra-ring etalon. The etalon can be designed with a free spectral range equal to the ITU grid spacing. However, mode pulling is still observed. Insertion of an acousto-optic frequency modulator in the fiber ring can reduce gain pulling. Multi-line performance can be further improved by placing a gain-flattening filter in the cavity to equalize the gain across the desired spectral range. However, most gain flattening filters are unable to sufficiently flatten the gain curve for obtaining a uniform laser emission intensity across the desired spectral range.
In addition to achieving multi-wavelength emission of uniform intensity, the individual emission lines (channels) propagating in the optical fiber still must be separated (demultiplexed) in order to imprint information on the various channels.
It would therefore be desirable to provide a single laser source module that serves as a source for all channels in the optical system. More particularly, it would be desirable to provide a multi-wavelength, gain equalized, demultiplexed laser source system based upon a single gain medium, wherein the channels can be modulated independently. It would also be desirable to multiplex the modulated channels into a single output fiber without the need for multi-fiber interconnections, which can simplify the system design and reduce costs. It would also be desirable to prevent channel drift by locking all channels to a single stable wavelength reference.
The invention relates to a modulatable external cavity fiber laser providing stable multi-wavelength laser emission. A grating multiplexer section of the laser defines a separate ring topology for each wavelength or channel. Individual mirrors can be placed at the focus of the grating imaging optics operating as parallel output couplers, with the reflectivity of mirrors being tailored to the fiber laser""s gain coefficients. The grating can be angle-tuned to different parts of the ITU grid.
According to an aspect of the invention, the multi-wavelength external cavity fiber laser includes a fiber-optic amplifier (FOA) optically coupled to an optical circulator (OC), wherein the FOA receives optical radiation having a plurality of wavelengths from a first port of the OC and supplies amplified optical radiation to a second port of the OC. An optical element, such as a focusing lens, is positioned with its focal plane at a third port of the OC to intercept optical radiation emerging from the third port. The fiber laser further includes a partially reflecting output coupler and a dispersive element positioned in an optical path between the optical element and the output coupler. The partially reflecting output coupler intercepts the plurality of output beams produced by the dispersive element, with each output beam having a unique wavelength. The partially reflecting output coupler and the FOA coupled to the OC form the external cavity of the fiber laser. The dispersive element can be a free space grating or an immersion grating made, for example, of silicon or another material with a suitable index of refraction (nxe2x89xa71.4) that is optically transparent in the wavelength range of interest for the application.
The reflectivity of the partially reflecting output coupler for each of the output beams is selected so as to flatten the optical gain of the FOA across the range of emission wavelengths.
The fiber laser further can include optical modulators, wherein each modulator is associated with one of the optical output beams and can modulate the intensity of the particular output beam.
Further embodiments may include one or more of the following features. An acousto-optic frequency shifter (AOFS) as well as a spectral filter, for example, a Fabry-Perot etalon, may be located in the ring topology. Also provided may be a wavelength locker which derives an input signal from at least one wavelength and controls the spectral filter, such as an etalon constructed to have a free spectral range equivalent to the ITU grid spacing, so as to collectively tune and lock each wavelength of the plurality of wavelengths based on the input signal from the at least one wavelength.
A beam combiner located outside the laser cavity can intercept the output modulated beams from the modulator and combine the modulated beams into an overlapping multi-wavelength beam. The beam combiner may include a second focusing element, such as a lens, and a second dispersive element, which can also be a free space grating or an immersion grating. Alternatively, the beam combiner may be a fiber coupler, a star coupler or an arrayed waveguide coupler.
Further features and advantages of the present invention will be apparent from the following description of preferred embodiments.